Transparent ceramics, manufacturing method thereof, and magneto-optical device

ABSTRACT

A transparent ceramic material is manufactured by molding a source powder into a compact, the source powder comprising a rare earth oxide consisting of at least 40 mol % of terbium oxide and the balance of another rare earth oxide, and a sintering aid, sintering the compact at a temperature T (1,300° C.≤T≤1,650° C.) by heating from room temperature to T1 (1200° C.≤T1≤T) at a rate of at least 100° C./h, and optionally heating from T1 at a rate of 1-95° C./h, and HIP treating the sintered compact at 1,300-1,650° C. The ceramic material has improved diffuse transmittance in the visible region and functions as a magneto-optical part in a broad visible to NIR region.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2018-043076 filed in Japan on Mar. 9,2018, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to a method for manufacturing a transparentceramic material in the form of a Tb-based complex oxide sintered bodyhaving a high transparency or light transmittance in the visible tonear-infrared (NIR) region, a transparent ceramic material manufacturedthereby, and a magneto-optical device.

BACKGROUND ART

There have been developed and manufactured a wide variety of ceramicscovering from traditional ceramics such as tiles and potteries to fineceramics such as piezoelectric parts, superconductor parts andtransparent ceramics.

Transparent ceramics draw attention as a new replacement to singlecrystals since laser oscillation by ceramics was reported in 1990s.Transparent ceramics are used in magneto-optical parts, scintillatormaterials, high-strength window members and the like, enable thecomposition design and size enlargement which are impossible with singlecrystals, and are regarded advantageous in manufacture cost over singlecrystals.

The magneto-optical part which is one of major applications oftransparent ceramics refers to a material having a Faraday effect ofpolarizing light by applying a magnetic field across the part, and isused, for example, in optical communications and fiber laser devices.The use of the magneto-optical part as a Faraday rotator is effectivefor restraining the return light occurring at the end surface of afiber, contributing to the stabilization of laser output. For example,garnet-type Tb₃Ga₅O₁₂ (Patent Document 1: JP 4878343), C-type (cubic)rare earth-based (Tb_(x)Re_(1-x))₂O₃ (Patent Documents 2, 3: JP 5704097,WO 2015/186656), pyrochlore-type Tb₂Hf₂O₇ (Patent Document 4: JP-A2016-169115) and the like are currently produced as ceramicmagneto-optical parts. Inter alia, the C-type rare earth-based oxide isa material worth expecting because it can contain much terbium ionshaving a great Faraday effect and thus contributes to a length reductionof magneto-optical parts.

Among the physical properties required for transparent ceramics used asoptical materials as well as magneto-optical parts, it is most importantto acquire a high transparency comparable to single crystals. The causefor exacerbating transparency is generally divided into two factors. Onefactor is the light absorption of the material itself, for example, thematerial may be colored due to impurities and crystal defects. The lightabsorption can be reduced by controlling impurities and optimizingpreparation conditions. The other factor is light scattering, that is,incident light is scattered in every direction, for example,exacerbating laser quality. The cause for light scattering includes thefollowing three types: (1) precipitation of a heterophase different fromthe major phase of the material (phase of main composition), (2) bubblesleft behind within the material, and (3) grain boundary scatteringoccurring at the boundary between crystal grains. Every scatteringfactor can be reduced by optimizing the heat treatment step at a hightemperature of at least 1,300° C., known as sintering step.

Among these, the grain boundary scattering is caused by a minutedifference in refractive index between crystal grains and referred to asRayleigh-Gans-Debye scattering (RGD scattering) dependent on thereciprocal of the square of wavelength. Non-Patent Document 1 (J. Am.Ceram. Soc., 86, 480 (2003)) pays attention to the size of crystalgrains in controlling the RGD scattering for the purpose of improvingthe transparency of hexagonal alumina transparent ceramic. Hexagonalalumina has a refractive index which is different by 0.008 at maximumdepending on crystal orientation, the difference of refractive index dueto orientation becomes the cause of RGD scattering. Since the RGDscattering depends on not only the difference of refractive index, butalso the grain size, the RGD scattering decreases as the grain sizebecomes smaller. It is demonstrated that when an attempt is made totransparentize hexagonal alumina ceramic by setting the preparationconditions such that the hexagonal alumina ceramic may have differentgrain size, an alumina ceramic having a high degree of transparency issynthesized as long as the grain size is up to 1 μm.

For the cubic structure which is common to transparent ceramics, it isunlikely that the difference of refractive index between grains becomeslarge. In the cubic structure, unlike the hexagonal structure, therefractive index does not depend on crystal orientation. It is thenbelieved that transparent ceramics composed solely of the main componentcause less RGD scattering.

However, of actual transparent ceramics, few ceramics are composedsolely of the main component, because additives such as sintering aidare mostly added. If the sintering aid and the main component oxide arenot uniformly mixed, a compositional variation occurs among grains,eventually bringing a difference of refractive index between grains toinduce RGD scattering. Particularly when the difference in ionic radiusbetween elements of the main component and the sintering aid exceeds15%, it is difficult to uniformly mix the main component and thesintering aid. In order to avoid this phenomenon, Patent Document 5(JP-A 2008-143726) sets the size of crystal grains to range from 5 μm to300 μm, for thereby achieving a consistent intragranular compositionwith as large crystal grains as possible. However, since the RGDscattering due to a difference of refractive index between grains isalso affected by grain size, it is not concluded that better results arealways obtainable from large crystal grains.

Patent Documents 2 and 3 disclose that C-type rare earth-basedtransparent ceramics are used in magneto-optical parts. In PatentDocument 2, a transparent ceramic having a transmittance of about 70% inthe NIR region is prepared by adding an oxide of Group 4 element orGroup 2 element to a source powder as sintering aid in an amount of 0.5%by weight based on the source powder, mixing and grinding. Because ofthe non-uniform distribution of the sintering aid, it is difficult toobtain a high transmittance of at least 80%. Addressing the problem,Patent Document 3 discloses that a transparent ceramic having aninsertion loss of up to 0.97 dB at 1,064 nm, i.e., a lineartransmittance of at least 80.3% is obtained by simultaneousco-precipitation of a sintering aid during synthesis of a source powder,for thereby achieving uniform distribution of the sintering aid.

Although a high linear transmittance is available in Patent Document 3,the diffuse transmittance (of light emerging from the emergent surfaceof an optical part, the quantity of light scattered in directions otherthan a perpendicular direction to the emergent surface, to be describedlater in detail) at wavelength 1,064 nm is, in most cases, 0.6% to 0.9%.The presumed reason is that since the temperature of HIP treatment inPatent Document 3 is as high as 1,800° C., the resulting transparentceramic is composed of crystals with a grain size of about 5 μm, and asa result, RGD scattering is increased. When the wavelength dependence ofdiffuse transmittance is confirmed in practice, it is found that acomponent depending on the reciprocal of the square of wavelength, thatis, RGD scattering accounts for about 90% of the overall diffusetransmittance. When the scattering transmittance is 0.6% to 0.9% atwavelength 1,064 nm, the material is at a practically acceptable levelas magneto-optical parts in the NIR region. When wavelength 532 nm whichis half of wavelength 1,064 nm is considered, because of dependence onthe reciprocal of the square of wavelength, the scattering transmittanceat wavelength 532 nm is 2.4% to 3.6%, which is 4 times the scatteringtransmittance at wavelength 1,064 nm. Then the ceramic is difficult touse as magneto-optical parts for light of wavelength 532 nm. Namely,further reduction of scattering is necessary for C-type rare earth-basedmagneto-optical parts to function over a wide wavelength range fromvisible to NIR region.

CITATION LIST

-   Patent Document 1: JP 4878343-   Patent Document 2: JP 5704097 (U.S. Pat. No. 9,470,915, EP 2687500)-   Patent Document 3: WO 2015/186656-   Patent Document 4: JP-A 2016-169115-   Patent Document 5: JP-A 2008-143726-   Non-Patent Document 1: J. Am. Ceram. Soc., 86, 480 (2003)-   Non-Patent Document 2: J. Am. Ceram. Soc., Discussions and Notes,    1972, February 109

SUMMARY OF INVENTION

An object of the invention is to provide a method for manufacturing atransparent ceramic material in the form of a Tb-based complex oxidesintered body having improved diffuse transmittance in the visibleregion and functioning as a magneto-optical part in a wide wavelengthrange from visible to NIR region; a transparent ceramic materialmanufactured thereby; and a magneto-optical device.

In one aspect, the invention provides a method for manufacturing atransparent ceramic material, comprising the steps of:

molding a source powder into a compact, the source powder comprising arare earth oxide consisting of terbium oxide and at least one other rareearth oxide selected from yttrium oxide, scandium oxide, and oxides oflanthanoid elements (exclusive of terbium), in a molar fraction of atleast 40 mol % terbium oxide and the balance of the other rare earthoxide as a main component, and an oxide of at least one element selectedfrom Group 2 elements and Group 4 elements as a sintering aid,

sintering the compact at a sintering temperature T wherein 1,300°C.≤T≤1,650° C., the sintering step including heating from roomtemperature to a predetermined temperature T1 wherein 1200° C. T1 T at aheating rate of at least 100° C./h, and optionally heating from thetemperature T1 at a rate of from 1° C./h to 95° C./h,

hot isostatic pressing (HIP) the sintered compact at 1,300° C. to 1,650°C., thereby forming the transparent ceramic material in the form of acomplex oxide sintered body represented by the formula (I):(Tb_(x)R_(1-x))₂O₃  (I)wherein x is a number: 0.4≤x≤1.0, and R is at least one other rare earthelement selected from yttrium, scandium, and lanthanoid rare earthelements (exclusive of terbium), the sintered body having a crystalgrain size of from 0.5 μm to 2 μm, wherein a specimen of 11 mm long madefrom the sintered body has an overall light transmittance of at least80.0% and a diffuse transmittance of up to 1.6% at wavelength 633 nm.

In a preferred embodiment, the source powder is obtained by furnishingan aqueous solution containing (a) terbium ions, (b) ions of at leastone rare earth element selected from yttrium, scandium, and lanthanoidelements (exclusive of terbium), and (c) ions of at least one elementselected from Group 2 elements and Group 4 elements, letting components(a), (b) and (c) co-precipitate from the solution, filtering and heatdrying the co-precipitate.

Typically, the sintering aid is zirconium oxide and/or hafnium oxide.Preferably, the sintering aid is added in an amount of 0.1% to 3% byweight.

Preferably, the other rare earth oxide is yttrium oxide.

Preferably, the predetermined temperature T1 is up to 1,400° C.

In a preferred embodiment, the heating rate in the sintering step fromroom temperature to the predetermined temperature T1 is up to 300° C./h.

In another aspect, the invention provides a transparent ceramic materialin the form of a complex oxide sintered body represented by the formula(I):(Tb_(x)R_(1-x))₂O₃  (I)wherein x is a number: 0.4≤x≤1.0, and R is at least one rare earthelement selected from yttrium, scandium, and lanthanoid elements(exclusive of terbium), the sintered body having a crystal grain size offrom 0.5 μm to 2 μm, wherein a specimen of 11 mm long made from thesintered body has an overall light transmittance of at least 80.0% and adiffuse transmittance of up to 1.6% at wavelength 633 nm.

In a preferred embodiment, the specimen of 11 mm long has an overalllight transmittance of at least 80.2% and a diffuse transmittance of upto 0.7% at wavelength 1,064 nm.

In a further aspect, the invention provides a magneto-optical devicecomprising a magneto-optical part using the transparent ceramic materialdefined herein.

Advantageous Effects of Invention

The method for manufacturing a transparent ceramic material in the formof a cubic (C type) rare earth complex oxide sintered body includes thesintering step and the HIP step, wherein the heating rate up to thepredetermined temperature T1 in the sintering step is high while thesintering temperature T is kept relatively low, and the HIP temperatureof the HIP step is kept relatively low. A sintered body containingcrystal grains with a grain size of up to 2 μm and having a density ofat least 95% is obtained, thereby reducing RGD scattering due to acompositional variation among crystal grains. The transparent ceramicmaterial functions as an optical part in a broad range from visible toNIR.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view of an optical isolator.

FIG. 2 is a diagram showing diffuse transmittance spectra of Example 1-1and Comparative Example 1-6.

DESCRIPTION OF PREFERRED EMBODIMENTS

Method for Manufacturing Transparent Ceramic Material

One embodiment of the invention is a method for manufacturing atransparent ceramic material, comprising the steps of:

molding a source powder into a compact, the source powder comprising arare earth oxide consisting of terbium oxide and at least one other rareearth oxide selected from yttrium oxide, scandium oxide, and oxides oflanthanoid elements (exclusive of terbium), in a molar fraction of atleast 40 mol % terbium oxide and the balance of the other rare earthoxide as a main component, and an oxide of at least one element selectedfrom Group 2 elements and Group 4 elements as a sintering aid,

sintering the compact at a sintering temperature T (1,300° C.≤T≤1,650°C.), the sintering step including heating from room temperature to apredetermined temperature T1 (1200° C.≤T1≤T) at a heating rate of atleast 100° C./h, and optionally heating from the temperature T1 at aheating rate of from 1° C./h to 95° C./h, hot isostatic pressing (HIP)the sintered compact at 1,300° C. to 1,650° C., thereby forming thetransparent ceramic material in the form of a complex oxide sinteredbody represented by the formula (I):(Tb_(x)R_(1-x))₂O₃  (I)wherein x is a number: 0.4≤x≤1.0, and R is at least one other rare earthelement selected from yttrium, scandium, and lanthanoid rare earthelements (exclusive of terbium). The sintered body has a crystal grainsize of from 0.5 μm to 2 μm. A specimen of 11 mm long made from thesintered body has an overall light transmittance of at least 80.0% and adiffuse transmittance of up to 1.6% at wavelength 633 nm.

Method

The method generally involves furnishing particles of preselected metaloxides as the source powder (starting raw material), press molding thesource powder into a compact of predetermined shape, effecting binderburnout, sintering the compact for consolidation to a relative densityof at least 95%, and hot isostatic pressing (HIP) the sintered body.This may be followed by post-treatment such as annealing.

Source Powder

As the source powder with which the inventive method starts, particlesof any metal oxides which form a sintered body of Tb-based complex oxidewhich exhibits light transmission are advantageously used. The rawmaterials may be selected in accordance with a particular method forpreparing the source powder, from rare earth metal powders containingterbium (Tb) and at least one rare earth metal selected from yttrium(Y), scandium (Sc), and lanthanoid elements (exclusive of terbium),aqueous solutions thereof in nitric acid, hydrochloric acid, andsulfuric acid, and nitrates, halides and sulfides containing terbium(Tb) and at least one rare earth element selected from yttrium (Y),scandium (Sc), and lanthanoid elements (exclusive of terbium). The rawmaterials may have a purity of market level (at least 3N, at least 99.9%by weight), preferably at least 4N (99.99% by weight), more preferablyat least 5N (99.999% by weight).

While a transparent ceramic material in the form of a Tb-based complexoxide sintered body is finally manufactured using oxide powders in thedesired mixing ratio as the source powder, the oxide powders used hereinpreferably have a primary particle size of up to 1,000 nm, morepreferably up to 300 nm. If the primary particle size exceeds 1,000 nm,the powder is less sinterable and sintering treatment must be performedat higher temperature, imposing severer restrictions to the preparationcost and apparatus. Also, if primary particles are large, greater gapsare defined among primary particles during molding, with the likelihoodof voids being left. The method of preparing a power having such aprimary particle size includes bottom-up synthesis methods such asco-precipitation, uniform precipitation and hydrothermal reaction, andtop-down synthesis methods such as mechanical treatment, typically ballmilling of coarse powder of about 100 μm, to pulverize particles to adesired primary particle size. In the practice of the invention, a rawmaterial synthesized by the bottom-up method is preferably used. Primaryparticles are preferably of spherical or plate shape, but not limitedthereto as long as transparentizing is possible.

The source powder used herein may further contain a sintering aidcomposed of an oxide of at least one element selected from Group 2elements, Group 4 elements, Group 13 elements and Group 14 elements. Thesource powder preferably contains a sintering aid composed of an oxideof Group 4 element, most preferably Zr oxide and/or Hf oxide. While thesintering aid has an impact on the subsequent sintering step, the typeand amount of the sintering aid may be optimized for transparentization.The amount of the sintering aid is preferably from 0.1% to 3% by weight,more preferably from 0.5% to 3% by weight, calculated as oxide of thesintering aid, based on the total weight of rare earth complex oxide(main component). Less than 0.1% by weight of the sintering aid may failto exert its effect whereas more than 3% by weight has the risk of thesintering aid precipitating out to become a scattering factor. Thesintering aid should preferably have a purity of at least 99.9% byweight. Most preferably, the sintering aid is added as ions during thebottom-up synthesis of the source powder, in order to make thecomposition uniform.

Also, in the source powder, any component other than the main component(oxides to form Tb-based complex oxide) and sintering aid shouldpreferably be controlled to to or below 10 ppm.

Preferably the source powder is prepared by the three-componentco-precipitation process to be described below.

Three-Component Co-Precipitation Process

The source powder is typically prepared by furnishing an aqueoussolution containing (a) terbium ions, (b) ions of at least one otherrare earth element selected from yttrium, scandium, and lanthanoidelements (exclusive of terbium), which is substantially non-absorptiveat wavelength 1.064 μm, and (c) ions of at least one element selectedfrom Group 2 elements and Group 4 elements, for example, titanium,zirconium, hafnium, calcium and magnesium ions, which forms an oxideserving as the sintering aid for preventing precipitation of aheterophase other than cubic crystals in the crystal structure ofterbium oxide-based complex oxide ceramic material, and lettingcomponents (a), (b) and (c) co-precipitate from the solution through theco-precipitation process. The resulting source powder (i.e., rare earthoxide powder to be sintered) contains a rare earth oxide consistingessentially of terbium oxide and at least one other rare earth oxideselected from yttrium oxide, scandium oxide, and oxides of lanthanoidelements (exclusive of terbium), in a molar fraction of at least 40 mol% of terbium oxide and the balance of other rare earth oxide, and thesintering aid consisting of the oxide of at least one element selectedfrom Group 2 and Group 4 elements. This process is designated herein asthree-component co-precipitation process.

As a typical three-component co-precipitation process, the oxide sourcepowder may be prepared by dissolving the predetermined raw materials sothat components (a), (b) and (c) are contained in an acidic aqueoussolution, for example, 5N nitric acid aqueous solution, adding analkaline aqueous solution, for example, ammonia to the aqueous solution,letting components (a), (b) and (c) co-precipitate as hydroxide,filtering the precipitate, and heat drying the hydroxide at atemperature of at least 500° C. The precipitate-inducing means is notlimited to the addition of an alkaline aqueous solution, and any desiredprecipitation means may be used as long as no disproportionation occursduring formation of co-precipitate of components (a), (b) and (c). Forexample, the method of adding oxalic acid to an acidic aqueous solutionhaving components (a), (b) and (c) dissolved therein and letting theoxalate precipitate, or the method of adding a salt containing carbonateions such as ammonium hydrogencarbonate or ammonium carbonate andletting the carbonate precipitate may be advantageously utilized.Further, the preferred treatment to obtain a powder of stable particleshape is, for example, by adding dropwise an alkaline aqueous solutionsuch as aqueous ammonia to an acidic aqueous solution containing ions ofthree components, to form hydroxide, adding dropwise an aqueous solutionof a carbonate ion-containing salt such as an ammonium hydrogencarbonateaqueous solution thereto, thereby once converting the hydroxide tocarbonate, aging the solution, and thereafter, adding dropwise aqueousammonia again for re-converting to hydroxide. With this treatment, theparticles precipitate in stable particle shape without agglomeration.

The raw material for component (a) is preferably terbium oxide (Tb₂O₃)powder having a purity of preferably at least 99% by weight, morepreferably at least 99.9% by weight or Tb₄O₇ powder having an equivalentpurity. Alternatively, a powder of another compound such as fluoride ornitride of terbium may be used as long as the compound is dissolved inan acidic aqueous solution to form terbium ions, but not complex ions.Of these, terbium oxide powder is more preferred because impurity ionscan have an impact on reaction or firing.

The raw material for component (b) is preferably at least one rare earthoxide powder selected from yttrium, scandium, and lanthanoid elements(exclusive of terbium), having a purity of preferably at least 99% byweight, more preferably at least 99.9% by weight. Alternatively, apowder of another compound such as fluoride or nitride of the rare earthelement may be used as long as the compound is dissolved in an acidicaqueous solution to form rare earth ions, but not complex ions. Ofthese, yttrium oxide, scandium oxide or lanthanoid oxide powder is morepreferred because impurity ions can have an impact on reaction orfiring.

The raw material for component (c) is preferably a powder of oxide of atleast one element selected from Group 2 and Group 4 elements, having apurity of preferably at least 99% by weight, more preferably at least99.9% by weight.

These raw materials are weighed in amounts corresponding to thecomposition of the transparent complex oxide sintered body finallyobtained therefrom, and dissolved in an acidic aqueous solution.Specifically, first the raw material for component (a) and the rawmaterial for component (b) are weighed in such amounts as to give thepredetermined molar ratio as rare earth oxides. Subsequently, the rawmaterial for component (c) is weighed in such an amount as to give thepredetermined content of its oxide relative to the total weight of rareearth oxides. Notably, when all the raw materials for components (a),(b) and (c) are oxides and completely dissolve in an acidic aqueoussolution, the ratio of the amounts of the raw materials as weighedcorresponds directly to the weight ratio (in parts by weight) in thesource powder obtained from the three-component co-precipitationprocess.

In dissolving the raw materials for components (a), (b) and (c) asweighed, the raw materials may be mixed together and the powder mixturebe dissolved in an acidic aqueous solution, or the raw materials may bedissolved in an acidic aqueous solution in sequence.

The acidic aqueous solution used herein is not particularly limited aslong as the raw materials for components (a), (b) and (c) aredissolvable therein without forming complex ions so that the solutionmay contain ions of components (a), (b) and (c). Examples include 5Nnitric acid aqueous solution, sulfuric acid aqueous solution, andhydrochloric acid aqueous solution. An acidic aqueous solution whichdissolves all the raw materials for three components completely ispreferred, with nitric acid solution being more preferred. The nitricacid solution has the additional advantage that the amount of inorganicsalts left after firing is small.

A precipitant may be added to the acidic aqueous solution containingions of three components whereby all ions of three componentsco-precipitate. The precipitant is not particularly limited as long asit may be removed from the co-precipitate by water washing andfiltration. Suitable precipitants include aqueous ammonia (NH₄OH),oxalic acid ((COOH)₂), ammonium hydrogencarbonate (NH₄HCO₃), andammonium carbonate ((NH₄)₂CO₃). Of these, aqueous ammonia is mostpreferred because impurity ions are minimized.

The oxide raw material obtained by the above procedure is a sourcepowder which contains a rare earth oxide consisting essentially of (A)terbium oxide and (B) at least one other rare earth oxide selected fromyttrium oxide, scandium oxide and lanthanoid oxides (exclusive ofterbium oxide), which is substantially non-absorptive at wavelength1.064 μm, in a molar fraction of at least 40 mol % of terbium oxide andthe balance of other rare earth oxide, and (C) the sintering aidconsisting of the oxide of at least one element selected from Group 2and Group 4 elements. The other rare earth oxide is preferably yttriumoxide.

The source powder itself is not significantly different fromconventional powder in physical properties such as particle shape andorientation, but characterized in that sintering reaction takes placeuniformly at a microscopic level.

The source powder thus prepared has a purity of preferably at least 99%by weight, and for use in the optical application, a purity of morepreferably at least 99.9% by weight, even more preferably at least99.99% by weight.

Provided that the sum of (A) terbium oxide and (B) other rare earthoxide is a molar ratio of 1 (100 mol %), the rare earth oxide consistsof at least 40 mol %, preferably from 40 mol % to 90 mol % of (A)terbium oxide and the balance of (B) other rare earth oxide. Preferablythe oxide source powder consists essentially of component (A), component(B) and component (C) or sintering aid.

A source powder based on (or composed mainly of) components (A) and (B)means that the total of oxides or components (A) and (B) is at least 90%by weight of the source powder. The total content of oxides orcomponents (A) and (B) is preferably at least 95% by weight, morepreferably at least 97% by weight.

Component (C) or sintering aid is an oxide of at least one elementselected from Group 2 and Group 4 elements.

The sintering aid is preferably a compound which prevents precipitationof a heterophase other than cubic crystals in the crystal structure ofterbium oxide-based complex oxide sintered body. Suitable sintering aidsinclude oxides of Group 4 elements such as titanium, zirconium andhafnium, and oxides of Group 2 elements such as magnesium and calcium. Amixture of Group 4 element oxide and Group 2 element oxide is alsoacceptable. Since these oxides are not absorptive around wavelength 1.06μm, they are suited for use in the transparent complex oxide sinteredbody of the invention. While the Group 4 element oxide is known as astabilizer in the sintering of yttria, it is also effective as astabilizer for the transparent complex oxide sintered body of theinvention. It is noted that Group 2 elements such as magnesium andcalcium have a high reaction activity because of strong ionic characterso that they are readily incorporated in rare earth oxides as solidsolution. Thus zirconium oxide and hafnium oxide are preferred as thesintering aid.

To the source powder, component (C) or sintering aid which preventsprecipitation of a heterophase other than cubic crystals in the crystalstructure of terbium oxide-based complex oxide sintered body, forexample, oxide of Group 4 element such as titanium, zirconium or hafniumor oxide of Group 2 element such as magnesium or calcium is preferablyadded in an amount of from 0.1% by weight to 3% by weight, morepreferably from 0.5% by weight to 3% by weight, based on the totalweight of the rare earth complex oxide (or main component). Outside therange, the resulting transparent complex oxide sintered body may fail tomeet one or more of the characteristics required as the magneto-opticaldevice material. For example, when a specimen of 11 mm thick is madefrom the sintered body, the insertion loss at wavelength 1,064 nminclusive of reflection loss at an end surface in thickness directionexceeds 0.99 dB.

It is noted that the Group 4 element oxide may be used alone or theGroup 2 element oxide may be used alone, or the Group 4 element oxideand the Group 2 element oxide may be simultaneously used as thesintering aid. Since the Group 4 element forms a tetravalent ion and theGroup 2 element forms a divalent ion, they are simultaneously added sothat the rare earth oxide is doped in the form to compensate for thevalence of total ions, or so-called charge compensation form, wherebyionic defects are minimized.

The source powder used herein should preferably have a primary particlesize of 100 to 1,000 nm, more preferably 100 to 300 nm. If the primaryparticle size is less than 100 nm, such powder is difficult to handle,for example, difficult to mold, and a compact, if molded, has a lowdensity and is susceptible to cracking because of a substantialshrinkage factor upon sintering. If the primary particle size exceeds1,000 nm, such powder is less sinterable and thus difficult to produce atransparent sintered body having a high density. The primary particlesize is determined as an average of lengths of 100 primary particles inan arbitrary field of view when observed under a scanning electronmicroscope or optical microscope.

Slurry Formation

For the purpose of facilitating handling and molding in the subsequentmolding step, the source powder is preferably converted to a slurry. Thesolvent used in the slurry may be selected from water and organicsolvents such as ethanol, and is not particularly limited as long asslurry dispersion is not adversely affected. An organic additive such asdispersant or binder may be added to the slurry to improve dispersionstability.

The amount of the organic additive added is preferably up to 5% byweight and more preferably a minimum amount above which the desiredphysical properties are obtainable. Also preferably, the slurry has asolid concentration of from 10% to 50% by weight. If the concentrationis less than 10% by weight, the slurry concentration is so low that theamount of the slurry for treatment may be inevitably increased, raisinga problem in productivity. If the concentration is more than 50% byweight, agglomeration may occur in the slurry, and the slurry may have ahigh viscosity and become difficult to handle. The treatment for forminga slurry may be ball mill mixing, ultrasonic dispersion, jet mill,homogenizer or other treatment, and not particularly limited as long asslurry formation is possible.

Molding

The source powder prepared as above is then molded. The molding step isgenerally divided into dry molding and wet molding and not particularlylimited as long as a compact can be transparentized, with the drymolding method being preferred from the aspect of productivity. The drymolding method involves removing the solvent from the slurry prepared inthe previous step, forming granules, and pressing the granules underpressure by uniaxial pressing or cold isostatic pressing (CIP) into acompact of predetermined size.

The step of removing the solvent from the slurry may be spray drying,vacuum drying or the like, and not particularly limited as long asgranules with a particle size of 50 to 500 μm are obtainable. If thegranule size is less than 50 μm, handling may be difficult and moldingbe poor. If the granule size is more than 500 μm, it may be difficult tocollapse gaps defined between granules during molding. Although thepressure of uniaxial pressing or CIP is not particularly limited, thepressing pressure may be set so as to achieve a relative density of atleast 50% of the true density of the material.

The resulting compact is heat treated at a temperature of from 250° C.to 800° C. to remove the organic additive added in the slurry formingstep. If the heating temperature is lower than 250° C., there is therisk of insufficient removal of the organic additive. If the heatingtemperature exceeds 800° C., there is the possibility of cracking orother failure. The removal of the organic additive is preferablyconfirmed by thermal analysis such as thermogravimetric/differentialthermal analysis (TG/DTA).

According to the invention, the compact as prepared above is sinteredand hot isostatic pressing (HIP) treated, yielding a transparent complexoxide sintered body (transparent ceramic material).

Sintering

In the manufacturing method of the invention, a heat sintering step ofresistance heating or induction heating mode may be advantageouslyutilized. The atmosphere is selected from vacuum (reduced pressure), areducing atmosphere such as hydrogen, and an oxidizing atmosphere suchas oxygen or air, and not particularly limited as long as the desiredrelative density is reached. In the case of resistance heating,sintering may be performed within an oxide container in order to avoidcontamination with impurities from the heater. The material of thecontainer is typically selected from alumina, yttria, zirconia and thelike, but not particularly limited.

In the sintering step of the manufacturing method of the inventionwherein the compact prepared as above is sintered by heating at asintering temperature T wherein 1,300° C.≤T≤1,650° C., heating from roomtemperature (e.g., from 1° C. to 40° C.) to a predetermined temperatureT1 wherein 1200° C.≤T1≤T is at a heating rate of at least 100° C./h, andin the case of further heating from the temperature T1 (i.e., T1<T),this heating is at a rate of from 1° C./h to 95° C./h, completing thesintering treatment.

Herein, the sintering temperature T is from 1,300° C. to 1,650° C.,preferably from 1,300° C. to 1,500° C. If T is lower than 1,300° C., thecompact is little sintered. If T is higher than 1,650° C.,inconveniently oxygen defects form in the material and become a lightabsorbing source. The sintering temperature in the range is effectivefor bringing the relative density as sintered to at least 95% of thetrue density of the inventive Tb-based complex oxide. If the relativedensity as sintered is less than 95% of the true density of thematerial, inconveniently transparentizing is not achieved by thesubsequent step or HIP treatment.

The sinter holding time may be optimized so as to reach the desiredrelative density and is preferably up to 20 hours, though not limitedthereto. If the sinter holding time is longer than 20 hours, undesirablythere is a more chance of oxygen defects forming in the material.

In order that the sintered body resulting from the sintering step reacha desired crystal grain size (from 0.5 μm to 2 μm) and a sintereddensity or relative density of at least 95%, it is important to controlthe heating rate during the sintering step. In general, the crystalgrain size is determined by the sintering temperature, i.e., the higherthe sintering temperature, the greater becomes the crystal grain size.Namely, sintering must be to terminated at a relatively low temperaturein order to maintain the sintered grain size small. However, a lowersintering temperature leads to a lower relative density after sintering,failing to reach the transparentizable density of 95%. The heating rateduring the sintering treatment has an impact on the sintered density. Inthe case of sintering at the same temperature, a higher heating rateresults in a higher sintered density. It is noted that if the heatingrate is high and the sintering temperature is high, grains grow sorapidly that bubbles in the sintered body are not fully eliminated, orbubbles coalesce into giant bubbles, becoming a scattering factor. Forthis reason, it is not a practice in the prior art to control theheating rate so high. Quite unexpectedly, the inventors have found thatwhen the sintering temperature T is set at a relatively low temperatureat which grain growth is not so promoted and the heating rate is setrelatively high, bubbles do not coalesce together and are rather fullyeliminated, and thus a transparent ceramic material of quality ismanufactured. That is, by selecting the sintering temperature at whichthe crystal grain size does not become so large and heating at a highrate up to the predetermined temperature T1 wherein 1,200° C.≤T1≤T(i.e., up to at least 1,200° C.), a high density is achievable withsmall crystal grains.

In the sintering step wherein the compact prepared as above is sinteredby heating at a sintering temperature T wherein 1,300° C.≤T≤1,650° C.,the heating rate from room temperature to the predetermined temperatureT1 wherein 1,200° C.≤T1≤T is at least 100° C./h, preferably from 100°C./h to 300° C./h.

The sintering temperature T and heating rate must be optimized inaccordance with the amount of the sintering aid. As long as the amountof the sintering aid is 0.1% to 3% by weight, crystal grains do not growlarge until the temperature rises up to 1,400° C. Then the heating rateis preferably accelerated until the temperature rises up to 1,400° C.That is, the predetermined temperature T1 is preferably from 1,200° C.to 1,400° C.

In the heating course, when the temperature exceeds 1,400° C.,preferably 1,200° C., it is recommended that the heating rate set tillthen is not continued. If the heating rate is set as high as 100 to 300°C./h in the high-temperature region in excess of 1,400° C., grain growthis promoted rather than bubble expelling, with the risk of bubbles beingtaken in grains. The bubbles left within grains during sintering are notremoved in the subsequent HIP treatment step, but permanently left inthe transparent ceramic material, becoming a light scattering factor.Therefore, during the sintering step, in a high-temperature region inexcess of 1,400° C., preferably in excess of 1,200° C., where graingrowth is promoted, the heating rate is preferably set slow or low inorder to control rapid grain growth and to promote bubble expelling.

It is thus preferable that the heating rate subsequent to thepredetermined temperature T1 is set slow or low. Specifically, theheating rate is preferably 1 to 95° C./h, more preferably 10 to 75°C./h, even more preferably 10 to 50° C./h. Namely, the heating rate ispreferably as low as possible within the acceptable range. Also, in atemperature range in excess of the predetermined temperature T1, aplurality of heating rates in the range of 1 to 95° C./h may becombined. In an example wherein T1=1,400° C., the heating profile from1,400° C. to 1,600° C. may be set such that the heating rate is 50° C./hfrom 1,400° C. to 1,500° C., 10° C./h from 1,500° C. to 1,550° C., and40° C./h from 1,550° C. to 1,600° C.

The foregoing sintering treatment yields a primary sintered body.

HIP

According to the inventive method, the sintering step is followed bypost-treatment, typically hot isostatic pressing (HIP). In order tofurther improve the transparency of the primary sintered body resultingfrom the precedent sintering step, HIP sintering is performed at a highpressure of from 100 MPa to 250 MPa and a temperature of from 1,300° C.to 1,650° C., preferably from 1,300° C. to 1,600° C., for 0.5 to 3hours. The temperature of the HIP treatment may be higher or lower thanthe sintering temperature and is not particularly limited as long astransparentizing is possible and the crystal grain size is up to 2 μm.

If the HIP pressure is lower than 100 MPa, bubbles are not fullyeliminated during the HIP treatment. A HIP pressure in excess of 250 MPacan cause failure of the apparatus. The pressure medium for the HIPtreatment is preferably an inert gas such as Ar or N₂, which may containup to 10% of oxygen. It is recommended that during the HIP treatment,the primary sintered body is received in a metal container such asmolybdenum, tungsten or iridium, or an oxide container such as aluminaor yttria because this prevents scattering of contaminants from theheater.

The above HIP treatment yields a secondary sintered body.

Additional Steps

In the manufacturing method of the invention, the transparent complexoxide to sintered body resulting from the sintering and HIP steps ispreferably subjected to optical polishing at opposite end surfaces on anaxis of optical utilization. The optical surface accuracy (planarity) ispreferably up to λ/8, more preferably up to λ/10 wherein λ=633 nm. Ifthe planarity exceeds λ/8, beam quality or the like may be exacerbatedbelow the acceptable level in the optical application. The planarity ofan optical surface may be measured at a transmitted wave surface orreflected wave surface.

The opposite end surfaces of the sintered body as optically polished arethen coated with an antireflective film which is designed to adjust thecenter wavelength to 1,064 nm. This ensures precise optical measurement.

The method of the invention is successful in manufacturing a transparentceramic material having a high overall light transmittance and a lowdiffuse transmittance over a broad wavelength range from shortwavelength (e.g., 633 nm) to NIR.

Transparent Ceramic Material

Another embodiment of the invention is a transparent ceramic material inthe form of a complex oxide sintered body represented by the formula(I):(Tb_(x)R_(1-x))₂O₃  (I)wherein x is a number: 0.4≤x≤1.0, and R is at least one rare earthelement selected from yttrium, scandium, and lanthanoid elements(exclusive of terbium), the sintered body having a crystal grain size offrom 0.5 μm to 2 μm, wherein a specimen of 11 mm long made from thesintered body has an overall light transmittance of at least 80.0% and adiffuse transmittance of up to 1.6% at wavelength 633 nm.

In formula (I), R is not particularly limited as long as it contains atleast one rare earth element selected from yttrium, scandium, lanthanum,europium, gadolinium, ytterbium, holmium, and lutetium, and it maycontain another element such as erbium or thulium. Most preferably Rconsists of at least one rare earth element selected from yttrium,scandium, lanthanum, europium, gadolinium, ytterbium, holmium, andlutetium, and does not contain another element. R may be a singleelement or a mixture of plural elements in an arbitrary ratio. Interalia, R is preferably yttrium, scandium or lutetium, most preferablyyttrium.

In formula (I), x is a number of from 0.4 to less than 1.0. That is, therare earth oxide of formula (I) contains at least 40 mol %, calculatedas a molar fraction, of Tb₂O₃. If x is less than 0.4, a high Verdetconstant is unavailable.

Preferably x is from 0.4 to 0.9, more preferably from 0.45 to 0.8. Avalue of x in the range is preferable because a high Verdet constant isavailable and transparency is excellent. A value of x of up to 0.9 ispreferable because cracking during cooling after crystal growth iscontrolled and clouding of crystals is suppressed.

The transparent ceramic material contains a sintering aid composed of anoxide of at least one element selected from Group 2 and Group 4 elementsas well as the rare earth oxide. The sintering aid is preferably anoxide of at least one element selected from titanium, zirconium,hafnium, calcium, and magnesium.

The sintering aid composed of such oxide is present in an amount of from0.1% to 3% by weight, more preferably from 0.5% to 3% by weight,calculated as oxide of sintering aid, based on the total weight of therare earth complex oxide (main component). If the amount is less than0.1% by weight, the sintering aid effect is not consistently exerted andthe insertion loss at wavelength 1,064 nm may exceed 0.99 dB. If theamount exceeds 3% by weight, the sintering aid will precipitate outsingly without forming a solid solution, causing laser beam scattering,and the insertion loss may exceed 0.99 dB.

The fired body of the invention is a sintered body manufactured by themethod for manufacturing a transparent ceramic material and preferably atransparent complex oxide sintered body having the compositionalformula: (Tb_(x)Y_(1-x))₂O₃ wherein x is a number: 0.4≤x≤0.9. If x isless than 0.4, a high Verdet constant is unavailable. Thus x ispreferably at least 0.4. If x is more than 0.9, the phase transition ofterbium is uncontrollable. Thus x is preferably up to 0.9.

The transparent ceramic material has a crystal grain size of from 0.5 μmto 2 μm, preferably from 0.6 μm to 1.9 μm, more preferably from 0.8 μmto 1.6 μm. The grain size in the range ensures that the transparentceramic material is transparent and restrains RGD scattering.

The crystal grain size of the transparent ceramic material refers to anaverage grain size of crystal grains in the secondary sintered bodyafter HIP treatment, and may be determined by directly observing apolished surface of the ceramic material under a microscope or the like,for example, a reflection electron image under electron microscope(SEM). If the judgment of grain size on a polished surface is difficult,the surface is subjected to thermal etching at 1,200° C. or 0.1M dilutehydrochloric acid treatment for grain boundaries to appear prominent.The crystal grain size (or diameter of grains) may be determinedaccording to the formula:D=1.56C/(MN)  (1)wherein C is the length of an arbitrary line drawn on a high-resolutionimage under SEM, N is the number of grains on the line, and M is themagnification of the image (see Journal of the American Ceramic SocietyDiscussions and Notes, 1972, February 109). Herein N is preferably atleast 10, more preferably at least 100.

A specimen of 11 mm long made from the transparent ceramic material hasan overall light transmittance of at least 80.0%, preferably at least80.1%, more preferably at least 80.3% at wavelength 633 nm. At the sametime, the specimen of 11 mm long made from the transparent ceramicmaterial has a diffuse transmittance of up to 1.6%, preferably up to1.2%, more preferably up to 1.1% at wavelength 633 nm.

Furthermore, a specimen of 11 mm long made from the transparent ceramicmaterial has an overall light transmittance of at least 80.2%,preferably at least 80.3%, more preferably at least 80.9% at wavelength1,064 nm. At the same time, the specimen of 11 mm long made from thetransparent ceramic material has a diffuse transmittance of up to 0.7%,preferably up to 0.6%, more preferably up to 0.5% at wavelength 1,064nm.

As used herein, the “overall light transmittance” and “diffusetransmittance” are measured on an optically polished specimen oftransparent ceramic material having a length (optical path length) of 11mm with reference to JIS K7105 (ISO 13468-2:1999). Specifically, anintegrating sphere is provided with an inlet opening and an outletopening for light passage. A sample is placed at the inlet opening. Areflector is placed at the outlet opening. Then all light emerging fromthe sample is detectable by the integrating sphere. A component ofoverall light rays other than those in traveling direction is detected,with the reflector at the outlet opening removed. A ratio of theintensity of the detected light to the intensity of incident light tothe sample is the diffuse transmittance. A ratio of the intensity of theemergent light detected without removing the reflector at the outletopening to the intensity of incident light to the sample is the overalllight transmittance.

Magneto-Optical Device

The transparent ceramic material of the invention is suited asmagneto-optical parts for use in magneto-optical devices. In particular,the ceramic material is advantageously used as a Faraday rotator of anoptical isolator which is used in a wavelength region of 0.5 to 1.1 μm,i.e., in the visible to NIR region.

FIG. 1 is a schematic cross-sectional view of an optical isolator as atypical magneto-optical device having a Faraday rotator. In FIG. 1, theoptical isolator 100 comprises a Faraday rotator 110 of the transparentceramic material, a polarizer 120 and an analyzer 130 of polarizingmaterial arranged upstream and downstream of the Faraday rotator 110. Inthe optical isolator 100, polarizer 120, Faraday rotator 110 andanalyzer 130 are arranged on an optical axis 112 in the order of120-110-130, and preferably, a magnet 140 is rested on at least one sidesurface of those components. The magnet 140 is preferably received in ahousing 150.

The isolator is preferably used in a fiber laser for a processingmachine. Specifically, it is suitably used to prevent the laser lightemitted by the laser component from being reflected back to thecomponent to make its oscillation unstable.

EXAMPLES

Examples and Comparative Examples are given below for furtherillustrating the invention although the invention is not limitedthereto.

Example 1 Example 1-1

Terbium oxide powder (Tb₄O₇, purity≥99.9 wt %) and yttrium oxide powder(Y₂O₃, purity≥99.9 wt %), both available from Shin-Etsu Chemical Co.,Ltd., were weighed in such amounts totaling to 50 g that a molar ratioof Tb:Y was 50:50. These powders and 0.2 g (corresponding to 0.5% byweight, calculated as oxide, of the sintering aid) of a zirconiaprecursor to serve as a sintering aid (compositional formula: ZrOCl₂,purity 99.9 wt %, by Daiichi Kigenso Kagaku Kogyo Co., Ltd.) weredissolved in 200 ml of an acidic solution (compositional formula: HNO₃,concentration: 43 wt %, by Wako Pure Chemical Corp.). Then a basicsolution (compositional formula: NH₃, concentration: 23 wt %, by WakoPure Chemical Corp.) was added dropwise to the solution whereupon asubstantially insoluble hydroxide salt precipitated. The hydroxide saltthus precipitated was collected on a Buchner funnel and fired in oxygenatmosphere at 1,000° C. for 3 hours, obtaining terbium-containing oxide(Tb_(0.5)Y_(0.5))₂O₃ particles.

The terbium-containing oxide particles was added to a predeterminedamount of ethanol, which was fed into a resinous pot together withzirconia balls (average diameter 2 mm, by Nikkato Co., Ltd.) as millingmedia, 0.1 g of polyoxyethylene alkyl ether dispersant (polyoxyethylenestearyl ether, by Wako Pure Chemical Corp.) and 1.0 g of polyvinylalcohol binder (JMR-10 L by Japan Vam & Poval Co., Ltd.) as organicadditives. The mixture was ball milled into a slurry. The ball millingtime was 20 hours. The slurry mixture was granulated by a spray dryingtechnique and sieved, obtaining granules of a predetermined size(average particle size 250 μm).

The resulting granules were molded in a uniaxial press mold and coldisostatic pressed (CIP) into a cylindrical compact having a diameter of7 mm and a height of 16 mm. The compact was subjected to binder burnoutby heating in air atmosphere at 600° C. for 20 hours. The heating ratewas 20° C./h.

The compact after binder burnout was sintered by heating in vacuum at1,500° C. for 2 hours, obtaining a primary sintered body. In thesintering step, temperature T1 was 1,300° C. That is, the heating ratefrom room temperature to 1,300° C. (heating rate up to temperature T1)was 100° C./h, and the heating rate to a higher temperature (heatingrate subsequent to temperature T1) was 20° C./h.

The primary sintered body was placed in a molybdenum container andsubjected to HIP treatment at 198 MPa and 1,500° C. for 3 hours,obtaining a secondary sintered body.

The thus obtained transparent ceramic material was ground and polishedto a length of 11 mm. The optical end surfaces of each sample weresubjected to final optical polishing at an optical surface accuracy ofλ/8 (wherein λ=633 nm) and coated with an antireflective film which wasdesigned so as to adjust the center wavelength to 1,064 nm.

The specimen thus obtained was measured for several properties.

Relative Density of Primary Sintered Body

The relative density (%) of the primary sintered body was determined bymeasuring the density of the primary sintered body by Archimedes'method, dividing the density by the true density, and multiplying thequotient by 100.

Average Grain Size of Secondary Sintered Body

The secondary sintered body was observed under SEM after opticalpolishing and thermal etching at 1,200° C. for grain boundaries toappear prominent. The grain size was calculated by the above-describedprocedure according to formula (1). The number of grains N is ≥10.

Overall Light Transmittance and Diffuse Transmittance

The secondary sintered body was determined for overall lighttransmittance and diffuse transmittance with reference to JIS K7105 (ISO13468-2:1999) by combining a commercial UV/Vis spectrometer (model V-670by JASCO Corp.) with an integrating sphere. The sphere was provided witha pinhole such that light was irradiated to a spot having a diameter of3 mm. Measurement was performed in the double beam mode using a halogenlamp as the light source, and a photomultiplier (wavelength<750 nm) anda PbS photoelectric cell (wavelength 750 nm) as the detector. Theoverall light transmittance and diffuse transmittance were measured atwavelengths 1,064 nm and 633 nm. For each set of conditions, fivespecimens were measured, from which an average value (two significantfigures, unit in percent) was computed and evaluated.

Insertion Loss

An insertion loss was computed from the overall light transmittance anddiffuse transmittance measured as above at wavelengths 1,064 nm and 633nm, according to the following formula (2).Insertion loss (dB)=−10 log [{(overall light transmittance, %)−(diffusetransmittance, %)}/100]  (2)

Verdet Constant

A ceramic sample is inserted into a center bore of aneodymium-iron-boron magnet of outer diameter 32 mm, inner diameter 6mm, and length 40 mm, and polarizers are fitted at both ends. By using ahigh-power laser (beam diameter 1.6 mm, IPG Photonics Japan Co., Ltd.),and letting a high-power laser beam of wavelength 1,064 nm enter boththe end surfaces, a Faraday rotation angle θ is determined. The Faradayrotation angle θ is an angle at which the maximum transmittance isobtained when the polarizer on the emergent side is rotated. A Verdetconstant V is determined from the following equation. The strength (H)of magnetic field applied to the sample is computed by simulation fromthe size of the measurement system, residual magnet flux density (Br),and coercive force (Hc).θ=V×H×LHerein θ is a Faraday rotation angle (min), V is a Verdet constant, H isthe strength of magnetic field (Oe), and L is the length of the Faradayrotator (=11 mm in this example).

Examples 1-2 to 1-10 and Comparative Examples 1-1 to 1-8

Secondary sintered body samples were manufactured under the sameconditions as in Example 1-1 except that at least any one of the heatingrate up to the predetermined temperature T1, sintering temperature T,and HIP temperature was changed. In Comparative Examples 1-3 and 1-4,the sintering temperature T was 1,200° C. and the heating rate up to1,200° C. was a constant rate of 100° C./h and 200° C./h, respectively.

The results of Examples 1-1 to 1-10 and Comparative Examples 1-1 to 1-8are tabulated in Tables 1 and 2. FIG. 2 is a diagram showing diffusetransmittance spectra of Example 1-1 and Comparative Example 1-6.

TABLE 1 Example 1-1 1-2 1-3 1-4 1-5 Raw Terbium Molar ratio (%) 50 50 5050 50 material oxide Rare earth Type Y₂O₃ Y₂O₃ Y₂O₃ Y₂O₃ Y₂O₃ oxideMolar ratio (%) 50 50 50 50 50 Sintering Type ZrO₂ ZrO₂ ZrO₂ ZrO₂ ZrO₂aid Content (wt %) 0.5 0.5 0.5 0.5 0.5 Steps Source powder preparation3-component co-precipitation Sintering Sintering temp. T 1,500 1,5001,500 1,500 1,500 (° C.) Temperature T1 (° C.) 1,300 1,300 1,300 1,3001,300 Heating up to T1 100 150 200 250 300 rate subsequent 20 20 20 2020 (° C./h) to T1 Time (hr) 2 2 2 2 2 Atmosphere vacuum vacuum vacuumvacuum vacuum HIP Temperature (° C.) 1,500 1,500 1,500 1,500 1,500Heating rate (° C./h) 300 300 300 300 300 Time (hr) 3 3 3 3 3 Pressure(MPa) 198 198 198 198 198 Evaluation Relative density (%) 95.8 96.2 96.897.6 98.2 results of primary sintered body Average grain size (μm) 1.21.2 1.2 1.2 1.2 of secondary sintered body Overall light   @ 633 nm80.72 80.99 80.79 80.70 80.68 transmittance @ 1,064 nm 80.94 81.14 81.1080.80 80.95 (%) Diffuse   @ 633 nm 0.93 1.01 0.99 1.08 1.06transmittance @ 1,064 nm 0.40 0.42 0.38 0.45 0.41 (%) Insertion loss   @633 nm 1.00 0.99 1.00 1.01 1.01 (dB) @ 1,064 nm 0.96 0.95 0.95 0.97 0.96Example 1-6 1-7 1-8 1-9 1-10 Raw Terbium Molar ratio (%) 50 50 50 50 50material oxide Rare earth Type Y₂O₃ Y₂O₃ Y₂O₃ Y₂O₃ Y₂O₃ oxide Molarratio (%) 50 50 50 50 50 Sintering Type ZrO₂ ZrO₂ ZrO₂ ZrO₂ ZrO₂ aidContent (wt %) 0.5 0.5 0.5 0.5 0.5 Steps Source powder preparation3-component co-precipitation Sintering Sintering temp. T 1,500 1,5001,300 1,400 1,400 (° C.) Temperature T1 (° C.) 1,300 1,300 1,300 1,3001,300 Heating up to T1 100 100 300 200 100 rate subsequent 20 20 — 20 20(° C./h) to T1 Time (hr) 2 2 2 2 2 Atmosphere vacuum vacuum vacuumvacuum vacuum HIP Temperature (° C.) 1,600 1,400 1,300 1,400 1,400Heating rate (° C./h) 300 300 300 300 300 Time (hr) 3 3 3 3 3 Pressure(MPa) 198 198 198 198 198 Evaluation Relative density (%) 96.0 96.0 95.495.4 95.1 results of primary sintered body Average grain size (μm) 1.90.9 0.6 0.8 0.8 of secondary sintered body Overall light   @ 633 nm80.76 80.81 80.53 80.78 80.80 transmittance @ 1,064 nm 80.83 81.11 80.6380.88 80.83 (%) Diffuse   @ 633 nm 1.14 1.01 1.10 1.16 1.19transmittance @ 1,064 nm 0.48 0.39 0.46 0.41 0.44 (%) Insertion loss   @633 nm 1.01 1.00 1.02 1.01 1.01 (dB) @ 1,064 nm 0.97 0.95 0.98 0.97 0.97

TABLE 2 Comparative Example 1-1 1-2 1-3 1-4 1-5 1-6 1-7 1-8 Raw TerbiumMolar ratio (%) 50 50 50 50 50 50 50 50 material oxide Rare earth TypeY₂O₃ Y₂O₃ Y₂O₃ Y₂O₃ Y₂O₃ Y₂O₃ Y₂O₃ Y₂O₃ oxide Molar ratio (%) 50 50 5050 50 50 50 50 Sintering Type ZrO₂ ZrO₂ ZrO₂ ZrO₂ ZrO₂ ZrO₂ ZrO₂ ZrO₂aid Content (wt %) 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Steps Source powderpreparation 3-component co-precipitation Sintering Sintering temp. T (°C.) 1,500 1,500 1,200 1,200 1,500 1,700 1,700 1,700 Temperature T1 (°C.) 1,300 1,300 1,200 1,200 1,300 1,300 1,300 1,300 Heating up to T1 5020 100 200 100 100 200 20 rate subsequent 20 20 — — 20 20 20 20 (° C./h)to T1 Time (hr) 2 2 2 2 2 2 2 2 Atmosphere vacuum vacuum vacuum vacuumvacuum vacuum vacuum vacuum HIP Temperature (° C.) 1,500 1,500 1,3001,300 1,800 1,800 1,800 1,800 Heating rate (° C./h) 300 300 300 300 300300 300 300 Time (hr) 3 3 3 3 3 3 3 3 Pressure (MPa) 198 198 198 198 198198 198 198 Evaluation Relative density (%) 93.8 88.2 84.2 85.1 95.899.1 99.6 96.8 results of primary sintered body Average grain size (μm)1.2 1.2 0.4 0.4 5.8 5.8 5.8 5.8 of secondary sintered body Overall light  @ 633 nm devitrified devitrified devitrified devitrified 79.28 80.2780.24 80.27 transmittance @ 1,067 nm devitrified devitrified devitrifieddevitrified 80.09 80.66 80.84 80.78 (%) Diffuse   @ 633 nm devitrifieddevitrified devitrified devitrified 2.90 1.93 2.44 1.93 transmittance @1,064 nm devitrified devitrified devitrified devitrified 1.2 0.68 0.860.61 (%) Insertion loss   @ 633 nm devitrified devitrified devitrifieddevitrified 1.17 1.06 1.09 1.06 (dB) @ 1,064 nm devitrified devitrifieddevitrified devitrified 1.03 0.97 0.97 0.96

It is evident from the above that in Examples 1-1 to 1-10 wherein theheating rate up to 1,300° C. during the sintering treatment is in arange of 100 to 300° C./h, samples having a fully high density and hightransparency (overall light transmittance of at least 80.5% at eitherwavelength 633 nm or 1,064 nm) were obtained even when the sinteringtemperature was as relatively low as 1,500° C.

By contrast, in Comparative Examples 1-1 and 1-2 wherein the heatingrate up to 1,300° C. is as low as 50° C./h and 20° C./h, samplesdevitrified without reaching a high density. When a multiplicity ofsamples were manufactured under these sintering conditions, some sampleswere transparent, but the yield of transparent samples was low.

In Comparative Examples 1-3 and 1-4 wherein the sintering temperature isas low as 1,200° C., samples devitrified.

In Comparative Examples 1-5 to 1-8 wherein HIP treatment is at 1,800° C.as in Patent Document 3, the overall light transmittance at wavelength633 nm was from 79.2 to 80.3%, indicating a certain degree oftransparency. However, since the secondary sintered body had crystalgrains grown to a size in excess of 5 μm, its diffuse transmittance atwavelength 633 nm increased to 1.93 to 2.90% and its insertion lossincreased to 1.06 dB or higher.

By contrast, in Examples 1-1 to 1-10, the secondary sintered body wastransparentized while its crystal grains were kept to a size of up to 2μm. Then, its diffuse transmittance at wavelength 633 nm was suppressedas low as 1% and as a result, its insertion loss was suppressed as lowas 1.02 dB or below. It also showed an overall light transmittance of atleast 80.5% and a diffuse transmittance of up to 1.2% at wavelength 633nm, and an overall light transmittance of at least 80.6% and a diffusetransmittance of up to 0.5% at wavelength 1,064 nm. With respect to theinsertion loss at wavelength 1,064 nm, no substantial difference isfound between Examples 1-1 to 1-10 and Comparative Examples 1-6 to 1-8.

Furthermore, as shown in FIG. 1, the diffuse transmittance of Example1-1 is lower than that of Comparative Example 1-6 at all wavelengths.

By synthesizing a transparent body from a secondary sintered body havinga crystal grain size of up to 2 μm as in Examples, scattering oftransmitted light inclusive of the visible region on short wavelengthside is suppressed. Thus a transparent ceramic material functioning as amagneto-optical part over a broad range from visible to NIR ismanufactured.

The magneto-optical effect of Example 1-1 was measured to find a Verdetconstant of 0.255 min/(Oe·cm). This value is about 2 times the value ofconventional material TGG (Tb₃Ga₅O₁₂). This suggests that the inventivematerial is fully useful as magneto-optical parts.

Example 2 Examples 2-1 to 2-22

Secondary sintered body samples were manufactured under the sameconditions as in Example 1-1 except that at least any one of the molarratio of Tb/Y, the type and amount of sintering aid was changed. Theywere similarly evaluated.

The results are shown in Tables 3 and 4.

TABLE 3 Example 2-1 2-2 2-3 2-4 2-5 Raw material Terbium Molar ratio (%)40 40 40 40 50 oxide Rare earth Type Y₂O₃ Y₂O₃ Y₂O₃ Y₂O₃ Y₂O₃ oxideMolar ratio (%) 60 60 60 60 50 Sintering Type ZrO₂ ZrO₂ HfO₂ HfO₂ ZrO₂aid Content (wt %) 1 3 1 3 1 Steps Source powder preparation 3-componentco-precipitation Sintering Sintering temp. T 1,500 1,600 1,400 1,5501,500 (° C.) Temperature T1 (° C.) 1,300 1,300 1,300 1,300 1,300 Heatingup to T1 100 100 100 100 100 rate subsequent 20 20 20 20 20 (° C./h) toT1 Time (hr) 2 2 2 2 2 Atmosphere vacuum vacuum vacuum vacuum vacuum HIPTemperature (° C.) 1,500 1,600 1,400 1,550 1,500 Heating rate (° C./h)300 300 300 300 300 Time (hr) 3 3 3 3 3 Pressure (MPa) 198 198 198 198198 Evaluation results Relative density (%) 96.8 95.4 96.6 96.1 96.2 ofprimary sintered body Average grain size (μm) 0.9 1.1 1.2 0.8 0.9 ofsecondary sintered body Overall light   @ 633 nm 80.19 80.31 80.52 80.3780.24 transmittance @ 1,064 nm 80.40 80.43 80.58 80.44 80.38 (%) Diffuse  @ 633 nm 0.94 1.06 0.91 1.12 0.99 transmittance @ 1,064 nm 0.42 0.450.41 0.46 0.40 (%) Insertion loss   @ 633 nm 1.01 1.01 0.99 1.01 1.01(dB) @ 1,064 nm 0.97 0.97 0.96 0.97 0.97 Example 2-6 2-7 2-8 2-9 2-10Raw material Terbium Molar ratio (%) 50 50 50 50 50 oxide Rare earthType Y₂O₃ Y₂O₃ Y₂O₃ Y₂O₃ Y₂O₃ oxide Molar ratio (%) 50 50 50 50 50Sintering Type ZrO₂ HfO₂ HfO₂ ZrO₂ HfO₂ aid Content (wt %) 3 1 3 0.1 0.1Steps Source powder preparation 3-component co-precipitation SinteringSintering temp. T 1,600 1,500 1,550 1,300 1,300 (° C.) Temperature T1 (°C.) 1,300 1,300 1,300 1,300 1,300 Heating up to T1 100 100 100 100 100rate subsequent 20 20 20 — — (° C./h) to T1 Time (hr) 2 2 2 2 2Atmosphere vacuum vacuum vacuum vacuum vacuum HIP Temperature (° C.)1,600 1,600 1,600 1,300 1,300 Heating rate (° C./h) 300 300 300 300 300Time (hr) 3 3 3 3 3 Pressure (MPa) 198 198 198 198 198 Evaluationresults Relative density (%) 95.9 97.8 96.1 96.8 96.1 of primarysintered body Average grain size (μm) 0.9 1.6 1.1 0.9 1.0 of secondarysintered body Overall light   @ 633 nm 80.39 80.51 80.18 80.43 80.54transmittance @ 1,064 nm 80.45 80.75 80.43 80.59 80.76 (%) Diffuse   @633 nm 1.14 0.89 1.11 0.99 0.92 transmittance @ 1,064 nm 0.47 0.40 0.450.42 0.41 (%) Insertion loss   @ 633 nm 1.01 0.99 1.02 1.00 0.99 (dB) @1,064 nm 0.97 0.95 0.97 0.96 0.95

TABLE 4 Example 2-11 2-12 2-13 2-14 2-15 2-16 Raw material Terbium Molarratio (%) 60 60 60 60 70 70 oxide Rare earth Type Y₂O₃ Y₂O₃ Y₂O₃ Y₂O₃Y₂O₃ Y₂O₃ oxide Molar ratio (%) 40 40 40 40 30 30 Sintering Type ZrO₂ZrO₂ HfO₂ HfO₂ ZrO₂ ZrO₂ aid Content (wt %) 1 3 1 3 1 3 Steps Sourcepowder preparation 3-component co-precipitation Sintering Sinteringtemp. T 1,500 1,600 1,400 1,550 1,500 1,600 (° C.) Temperature T1 (° C.)1,300 1,300 1,300 1,300 1,300 1,300 Heating up to T1 100 100 100 100 100100 rate subsequent 20 20 20 20 20 20 (° C./h) to T1 Time (hr) 2 2 2 2 22 Atmosphere vacuum vacuum vacuum vacuum vacuum vacuum HIP Temperature(° C.) 1,550 1,600 1,600 1,600 1,500 1,600 Heating rate (° C./h) 300 300300 300 300 300 Time (hr) 3 3 3 3 3 3 Pressure (MPa) 198 198 198 198 198198 Evaluation results Relative density (%) 96.2 95.4 95.9 95.2 96.296.1 of primary sintered body Average grain size (μm) 0.8 1.1 1.9 1.21.1 0.8 of secondary sintered body Overall light   @ 633 nm 80.39 80.1980.55 80.47 80.58 80.19 transmittance @ 1,064 nm 80.59 80.46 80.79 80.6380.78 80.44 (%) Diffuse   @ 633 nm 0.96 1.12 0.93 1.04 0.96 1.12transmittance @ 1,064 nm 0.42 0.48 0.44 0.46 0.43 0.46 (%) Insertionloss   @ 633 nm 1.00 1.02 0.99 1.00 0.99 1.02 (dB) @ 1,064 nm 0.96 0.970.95 0.96 0.95 0.97 Example 2-17 2-18 2-19 2-20 2-21 2-22 Raw materialTerbium Molar ratio (%) 70 70 80 80 80 80 oxide Rare earth Type Y₂O₃Y₂O₃ Y₂O₃ Y₂O₃ Y₂O₃ Y₂O₃ oxide Molar ratio (%) 30 30 20 20 20 20Sintering Type HfO₂ HfO₂ ZrO₂ ZrO₂ HfO₂ HfO₂ aid Content (wt %) 1 3 1 31 3 Steps Source powder preparation 3-component co-precipitationSintering Sintering temp. T 1,400 1,550 1,500 1,600 1,400 1,550 (° C.)Temperature T1 (° C.) 1,300 1,300 1,300 1,300 1,300 1,300 Heating up toT1 100 100 100 100 100 100 rate subsequent 20 20 20 20 20 20 (° C./h) toT1 Time (hr) 2 2 2 2 2 2 Atmosphere vacuum vacuum vacuum vacuum vacuumvacuum HIP Temperature (° C.) 1,450 1,600 1,600 1,650 1,400 1,550Heating rate (° C./h) 300 300 300 300 300 300 Time (hr) 3 3 3 3 3 3Pressure (MPa) 198 198 198 198 198 198 Evaluation results Relativedensity (%) 95.8 95.2 96.8 96.1 96.4 96.1 of primary sintered bodyAverage grain size (μm) 1.1 1.6 1.4 1.1 0.9 1.1 of secondary sinteredbody Overall light   @ 633 nm 80.16 80.12 80.06 80.16 80.36 80.43transmittance @ 1,064 nm 80.59 80.43 80.41 80.44 80.59 80.67 (%) Diffuse  @ 633 nm 0.91 1.05 0.99 1.10 0.93 1.18 transmittance @ 1,064 nm 0.420.45 0.43 0.46 0.42 0.46 (%) Insertion loss   @ 633 nm 1.01 1.02 1.021.02 1.00 1.01 (dB) @ 1,064 nm 0.96 0.97 0.97 0.97 0.96 0.96

As seen from the above, the invention ensures that a transparent body issynthesized from a secondary sintered body having a crystal grain sizeof up to 2 μm, independent of the molar ratio of complex oxide and thetype and amount of sintering aid, which vary in the predetermined rangesto constitute the sintered body. The diffuse transmittance at wavelength633 nm is suppressed as low as 1% and the insertion loss is reduced to1.02 dB or below. The sample also shows an overall light transmittanceof at least 80.0% and a diffuse transmittance of up to 1.2% atwavelength 633 nm, and an overall light transmittance of at least 80.4%and a diffuse transmittance of up to 0.5% at wavelength 1,064 nm.According to the invention, a transparent ceramic material functioningas a magneto-optical part over a broad range from visible to NIR ismanufactured.

Example 3 Examples 3-1 to 3-4

Secondary sintered body samples were manufactured under the sameconditions as in Example 2-7 except that the heating rate and sinteringtemperature in the sintering step were changed. They were similarlyevaluated.

The results are shown in Table 5.

TABLE 5 Example 3-1 3-2 3-3 3-4 Raw Terbium oxide Molar ratio (%) 50 5050 50 material Rare earth oxide Type Y₂O₃ Y₂O₃ Y₂O₃ Y₂O₃ Molar ratio (%)50 50 50 50 Sintering aid Type HfO₂ HfO₂ HfO₂ HfO₂ Content (wt %) 1 1 11 Steps Source powder preparation 3-component co-precipitation SinteringSintering temperature T 1,400 1,300 1,500 1,600 (° C.) Temperature T1 (°C.) 1,300 1,300 1,300 1,300 Heating rate (° C./h) up to T1 200 300 200200 subsequent to T1 20 — 20 20 Time (hr) 2 2 2 2 Atmosphere vacuumvacuum vacuum vacuum HIP Temperature (° C.) 1,600 1,600 1,600 1,600Heating rate (° C./h) 300 300 300 300 Time (hr) 3 3 3 3 Pressure (MPa)198 198 198 198 Evaluation Relative density (%) of primary sintered body95.3 96.2 96.8 97.6 results Average grain size (μm) of secondarysintered body 1.6 1.6 1.6 1.6 Overall light transmittance (%)   @ 633 nm80.09 80.29 80.07 80.29 @ 1,064 nm 80.77 80.78 80.59 80.43 Diffusetransmittance (%)   @ 633 nm 1.02 1.04 1.00 1.12 @ 1,064 nm 0.42 0.430.42 0.45 Insertion loss (dB)   @ 633 nm 1.02 1.01 1.02 1.02 @ 1,064 nm0.95 0.95 0.96 0.97

As seen from the above, the invention ensures that a transparent body issynthesized from a secondary sintered body having a crystal grain sizeof up to 2 μm, even when the heating rate and sintering temperature inthe sintering step are changed within the predetermined range. Thediffuse transmittance at wavelength 633 nm is suppressed as low as 1%and the insertion loss is reduced to 1.02 dB or below. The sample alsoshows an overall light transmittance of at least 80.0% and a diffusetransmittance of up to 1.2% at wavelength 633 nm, and an overall lighttransmittance of at least 80.4% and a diffuse transmittance of up to0.5% at wavelength 1,064 nm. According to the invention, a transparentceramic material functioning as a magneto-optical part over a broadrange from visible to NIR is manufactured.

Example 4 Examples 4-1 to 4-3 and Comparative Examples 4-1, 4-2

Secondary sintered body samples were manufactured under the sameconditions as in Example 1-1 except that the temperature T1 in thesintering step was changed in the range of 1,000° C. to 1,500° C. Theywere similarly evaluated.

The results are shown in Table 6.

TABLE 6 Comparative Example Example 4-1 4-2 4-1 4-2 4-3 Raw Terbiumoxide Molar ratio (%) 50 50 50 50 50 material Rare earth oxide Type Y₂O₃Y₂O₃ Y₂O₃ Y₂O₃ Y₂O₃ Molar ratio (%) 50 50 50 50 50 Sintering aid TypeZrO₂ ZrO₂ ZrO₂ ZrO₂ ZrO₂ Content (wt %) 0.5 0.5 0.5 0.5 0.5 Steps Sourcepowder preparation 3-component co-precipitation Sintering Sinteringtemperature T (° C.) 1,500 1,500 1,500 1,500 1,500 Temperature T1 (° C.)1,000 1,100 1,200 1,400 1,500 Heating rate (° C./h) up to T1 100 100 100100 100 subsequent to T1 20 20 20 20 — Time (hr) 2 2 2 2 2 Atmospherevacuum vacuum vacuum vacuum vacuum HIP Temperature (° C.) 1,500 1,5001,500 1,500 1,500 Heating rate (° C./h) 300 300 300 300 300 Time (hr) 33 3 3 3 Pressure (MPa) 198 198 198 198 198 Evaluation Relative density(%) of primary sintered body 90.5 92.6 94.4 97.2 98.2 results Averagegrain size (μm) of secondary sintered body 1.4 1.4 1.4 1.4 1.7 Overalllight transmittance (%)   @ 633 nm devitrified devitrified 80.16 80.3980.08 @ 1,064 nm devitrified devitrified 80.44 80.45 80.25 Diffusetransmittance (%)   @ 633 nm devitrified devitrified 1.09 1.14 1.56 @1,064 nm devitrified devitrified 0.46 0.47 0.62 Insertion loss (dB)   @633 nm devitrified devitrified 1.02 1.01 1.05 @ 1,064 nm devitrifieddevitrified 0.97 0.97 0.99

As seen from the above, when the temperature T1 at which the heatingrate during the sintering step is maintained high (100° C./h) is lowerthan 1,200° C., the primary sintered body has a low relative density anddevitrifies. When the temperature T1 at which the heating rate ismaintained high (100° C./h) is at least 1,200° C., the primary sinteredbody is consolidated to a high density and transparentized. The sinteredbody shows an overall light transmittance of at least 80.0% and adiffuse transmittance of up to 1.6% at wavelength 633 nm, and an overalllight transmittance of at least 80.2% and a diffuse transmittance of upto 0.7% at wavelength 1,064 nm. When the temperature T1 was 1,500° C.(Example 4-3), the diffuse transmittances at wavelength 633 nm and 1,064nm marked slight increases over those when the temperature T1 is 1,200°C. or 1,400° C. (Examples 4-1 and 4-2).

Example 5 Examples 5-1 to 5-6

Secondary sintered body samples were manufactured under the sameconditions as in Example 1-1 except that the temperature T1 in thesintering step and/or the heating rate subsequent to temperature T1 waschanged. They were similarly evaluated. It is noted that in Example 5-4,the heating rate subsequent to temperature T1 was 50° C./h in a rangefrom 1,300° C. to 1,400° C., 10° C./h in a range from more than 1,400°C. to 1,450° C., and 20° C./h in a range from more than 1,450° C. to1,500° C. This heating rate is designated “zigzag” in Table 7.

The results are shown in Table 7.

TABLE 7 Example 5-1 5-2 5-3 5-4 5-5 5-6 Raw Terbium oxide Molar ratio(%) 50 50 50 50 50 50 material Rare earth oxide Type Y₂O₃ Y₂O₃ Y₂O₃ Y₂O₃Y₂O₃ Y₂O₃ Molar ratio (%) 50 50 50 50 50 50 Sintering aid Type ZrO₂ ZrO₂ZrO₂ ZrO₂ ZrO₂ ZrO₂ Content (wt %) 0.5 0.5 0.5 0.5 0.5 0.5 Steps Sourcepowder preparation 3-component co-precipitation Sintering Sinteringtemperature T (° C.) 1,500 1,500 1,500 1,500 1,500 1,500 Temperature T1(° C.) 1,300 1,300 1,300 1,300 1,400 1,400 Heating rate (° C./h) up toT1 100 100 100 100 100 100 subsequent to T1 50 30 10 zigzag 50 10 Time(hr) 2 2 2 2 2 2 Atmosphere vacuum vacuum vacuum vacuum vacuum vacuumHIP Temperature (° C.) 1,500 1,500 1,500 1,500 1,500 1,500 Heating rate(° C./h) 300 300 300 300 300 300 Time (hr) 3 3 3 3 3 3 Pressure (MPa)198 198 198 198 198 198 Evaluation Relative density (%) of primarysintered body 96.7 96.1 94.9 96.3 95.4 95.1 results Average grain size(μm) of secondary sintered body 1.4 1.4 1.1 1.2 1.4 1.2 Overall lighttransmittance (%)   @ 633 nm 80.24 80.15 80.29 80.14 80.21 80.28 @ 1,064nm 80.31 80.38 80.34 80.39 80.30 80.41 Diffuse transmittance (%)   @ 633nm 1.17 1.08 1.04 1.02 1.14 1.05 @ 1,064 nm 0.47 0.48 0.44 0.41 0.480.43 Insertion loss (dB)   @ 633 nm 1.02 1.02 1.01 0.99 1.02 1.00 @1,064 nm 0.99 0.98 0.98 0.97 0.99 0.97

As seen from the above, when the heating rate subsequent to temperatureT1 during the sintering step is lower than 100° C./h, specifically10-50° C./h, a transparent ceramic material having an insertion loss ofup to 1.02 dB at wavelength 633 nm and high transparency and functioningas a magneto-optical part over a broad range from visible to NIR ismanufactured. As seen from Example 5-4, even when the heating ratesubsequent to temperature T1 is switched plural times in the heatingstep within the range of 10° C./h to 50° C./h, the insertion loss atwavelength 633 nm is up to 1.02 dB. The ceramic material shows anoverall light transmittance of at least 80.1% and a diffusetransmittance of up to 1.2% at wavelength 633 nm, and an overall lighttransmittance of at least 80.3% and a diffuse transmittance of up to0.5% at wavelength 1,064 nm. According to the invention, a transparentceramic material having high transparency and functioning as amagneto-optical part over a broad range from visible to NIR ismanufactured.

Japanese Patent Application No. 2018-043076 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

The invention claimed is:
 1. A method for manufacturing a transparentceramic material, comprising the steps of: molding a source powder intoa compact, the source powder comprising a rare earth oxide consisting ofterbium oxide and at least one other rare earth oxide selected fromyttrium oxide, scandium oxide, and oxides of lanthanoid elements(exclusive of terbium), in a molar fraction of at least 40 mol % terbiumoxide and the balance of the other rare earth oxide as a main component,and an oxide of at least one element selected from Group 2 elements andGroup 4 elements as a sintering aid, sintering the compact at asintering temperature T wherein 1,300° C.≤T≤1,650° C., wherein in theheating course to the sintering temperature T the sintering stepincludes heating from room temperature to a predetermined temperatureT1, where 1200° C.≤T1≤T at a heating rate of at least 100° C./h, andoptionally heating from the temperature T1 at a rate of from 1° C./h to95° C./h, hot isostatic pressing (HIP) the sintered compact at 1,300° C.to 1,650° C., thereby forming the transparent ceramic material in theform of a complex oxide sintered body represented by the formula (I):(Tb_(x)R_(1-x))₂O₃  (I) wherein x is a number: 0.4≤x<1.0, and R is atleast one other rare earth element selected from yttrium, scandium, andlanthanoid rare earth elements (exclusive of terbium), the sintered bodyhaving a crystal grain size of from 0.5 μm to 2 μm, wherein a specimenof 11 mm long made from the sintered body has an overall lighttransmittance of at least 80.0% and a diffuse transmittance of up to1.6% at wavelength 633 nm.
 2. The method of claim 1 wherein the sourcepowder is obtained by furnishing an aqueous solution containing (a)terbium ions, (b) ions of at least one rare earth element selected fromyttrium, scandium, and lanthanoid elements (exclusive of terbium), and(c) ions of at least one element selected from Group 2 elements andGroup 4 elements, letting components (a), (b) and (c) co-precipitatefrom the solution, filtering and heat drying the co-precipitate.
 3. Themethod of claim 1 wherein the sintering aid is zirconium oxide and/orhafnium oxide.
 4. The method of claim 1 wherein the sintering aid isadded in an amount of 0.1% to 3% by weight.
 5. The method of claim 1wherein the other rare earth oxide is yttrium oxide.
 6. The method ofclaim 1 wherein the predetermined temperature T1 is up to 1,400° C. 7.The method of claim 1 wherein the heating rate in the sintering stepfrom room temperature to the predetermined temperature T1 is up to 300°C./h.
 8. The method of claim 1 wherein the sintering step includesheating from the temperature T1, where T1<T at a heating rate of from 1°C./h to 95° C./h.
 9. The method of claim 1 wherein the sintering stepincludes heating from the temperature T1, where T1<T at a heating rateof from 10° C./h to 50° C./h.
 10. The method of claim 1 wherein thesintering temperature T is up to 1,500° C.
 11. The method of claim 1wherein the sintering temperature T is at least 1,400° C.
 12. The methodof claim 1 wherein the HIP temperature is from 1,300° C. to 1,600° C.13. The method of claim 1 further comprising a step of heat treating thecompact at 250° C. to 800° C. to remove an organic additive included inthe compact before the sintering step.